1. Field of the Invention
The present invention generally relates to an information recording medium such as an optical disc, for recording video signals, voice signals, etc., therein. More particularly, the invention relates to a layer structure of an information recording medium in which information is recorded into a plurality of recording layers, which are laminated with intermediate layers being interlayered therebetween. Further, the invention relates to a reproducing device capable of reproducing the recorded information from the information recording medium.
2. Description of the Related Art
In recent years, in the field of the optical disc, how to increase a recording density for each side of the optical disc is under investigation. A double layer DVD having two recording layers is a typical example of such. A layer structure of the double layer DVD is shown in FIG. 9.
The DVD shown in FIG. 9 is formed with a first substrate 1 having a recording layer 2, a first reflecting layer 3, an intermediate layer 4 having a recording layer 5, a second reflecting layer 6, and a protecting layer 7. In FIG. 9, the first substrate 1 is made of a material permitting light to pass therethrough, such as PC (polycarbonate). The recording layer 2 having a first pit train for information recording is layered on the first substrate 1. The first reflecting layer 3 as an intermediate reflecting layer, which is made of gold (Au) or silicon carbide (SiC), is formed on the recording layer 2. The intermediate layer 4 is layered on the first reflecting layer 3. The intermediate layer 4 is made of photo-setting resin, such as ultraviolet curing acrylic resin, and has the recording layer 5 on the surface thereof, which is opposite to its surface being in contact with the first reflecting layer 3. The recording layer 5 has a second pit train for information recording.
The second reflecting layer 6 is formed on the recording layer 5 of the intermediate layer 4 by deposition process or the like. The second reflecting layer is formed with a reflecting layer made of aluminum (Al), which is higher in reflectivity than the intermediate reflecting layer. The protecting layer 7 of about 600 xcexcm thick and made of resin or the like is layered on the second reflecting layer 6. The protecting layer 7 is formed, for example, in a manner that the surface of the second reflecting layer 6 is coated with resin coating or the like. Pits each having a size corresponding to a wavelength of 650 nm carrying information are spirally or concentrically formed on each recording layer according to a given modulation method.
To reproduce information from the recording layer 2, the first reflecting layer 3 must permit some quantity of a laser beam to transmit therethrough. For this reason, the first reflecting layer 3 is made of a material having some degree of reflectivity and some degree of transmissivity, and is generally formed with a gold (Au) film of about 15 nm thick, or the like.
Information recorded in the double layer disc maybe played back or reproduced by a disc play-back or reproducing device. To play back the information, a laser beam converged by an objective lens, which is contained in a pickup optical system, is irradiated from the first substrate 1 of the rotating DVD toward the upper or lower recording layer. A reflecting light which is reflected by the reflecting layer (or intermediate reflecting film) formed on the recording layer) formed on the recording layer and is optically modulated by pits, grooves or the like, is received again through the objective lens, and converted into an electrical signal, and in this way a readout signal is formed. By using the readout signal, the information born by the recording layers is reproduced. Accordingly, when the DVD whose recording layer consists of double layers is compared with the DVD whose recording layer consists of a single layer, the former may have an increased recording capacity per one disc.
As described above, in the optical disc, its recording capacity per disc may be increased as the number of the recording layers increases. However, with increase of the number of the recording layers, the number of the intermediate layers also increases. To read information from a multi-layer disc having a number of recording layers, a laser beam passes through the intermediate layers. By the intermediate layers, a spherical aberration tends to occur. A reflecting light coming from the recording layer located far from the first substrate 1 contains a great spherical aberration component. This results in incorrect reproduction of information.
To lessen the adverse effect on the reflecting lights from the respective intermediate layers by the spherical aberration, each intermediate layer must be formed so as to have a thickness as thin as possible.
One of possible solutions to the above problem is to manufacture the multi-layer disc whose intermediate layer interlayered between the adjacent recording layers is extremely thin, e.g., about 3 xcexcm. Such a disc allows information to exactly be read out of each recording layer without giving rise to an interlayer cross talk when the method disclosed in JP-A-11-242824 is used.
In actually manufacturing the multi-layer disc, a transparent sheet of a fixed thickness is sandwiched between the recording layers, and those layers are bonded together, whereby an intermediate layer is formed between the recording layers. And a space between the adjacent recording layers (interlayer distance) is determined by a thickness of the intermediate layer. Only the sum of the thickness of the transparent sheet and the bonding layer for bonding the recording layers reaches the target interlayer distance (e.g., 3 xcexcm). Accordingly, it is difficult to manufacture a multi-layer disc having a small and stable space between the recording layers.
From the above background reasons, the present invention is to provide a multi-layer information recording medium in which a layer, e.g., an intermediate layer, formed between a multiple of recording layers is stably formed having a uniform predetermined height over the entire area of the layer, and a reproducing device which is capable of faithfully reproducing information from the recording layers.
To achieve the above object, there is provided a first information recording medium which comprises a substrate, and one or a plurality of layers formed on the substrate, wherein at least one of the layers is formed by mixing a base material and a hard material which has a predetermined thickness and determines a predetermined thickness of the layer.
In the first recording medium, the hard material of at least one of the layers is sandwiched between it and the adjacent substrate and a reflecting layer of another layer, so that the layer is stably formed while having a thickness equal to the height of the hard material over the entire layer.
In a second information recording medium, which depends from the first recording medium, the hard material contains spherical particles each of which has a predetermined outside diameter.
In the second recording medium, the hard material of the layer consisting of the hard material and the base material is sandwiched between it and the adjacent substrate and a reflecting layer of another layer, so that the layer is stably formed while having a thickness equal to the outside diameter of each particle contained in the hard material over the entire layer.
In a third information recording medium, which depends from the first recording medium, the hard material contains particles each configured like a column to have an outside diameter of a predetermined height.
In the third recording medium, the side face of a column-like material of the hard material of the layer consisting of the hard material and the base material is sandwiched between it and the adjacent layer or the reflecting layer of another layer, so that the layer is stably formed while having a thickness equal to the outside diameter of the column-like material over the entire layer.
In a fourth information recording medium, which depends from any of the first to third recording media, the hard material and the base material each consist of a transparent medium.
In the fourth recording medium, the layer consisting of the hard material and the base material is transparent and has a predetermined thickness over the entire layer. Accordingly, even when a pickup of the reproduction device projects a laser beam converted by an objective lens to the recording layer through the substrate and the layer, it can receives a sufficient amount of reflecting light from the recording layer. Therefore, it can stably generate a readout signal based on the information read out of the information recording layer.
In a fifth information recording medium, which depends from any of the first to fourth recording media, the layer consisting of the hard material and the base material is an intermediate layer sandwiched between two recording layers.
In the fifth recording medium, the hard material of a predetermined height, which is contained in the intermediate layer, is formed while being sandwiched between the two recording layers. Accordingly, the intermediate layer is stably formed having a predetermined thickness over the entire layer.
In a sixth information recording medium, which depends from any of the first to fifth recording media, the hard material consists of a medium whose refractive index is substantially equal to that of the base material.
In the sixth recording medium, the layer consisting of the hard material and the base material is uniform over the entire layer in refractive index and thickness. When the pickup of the reproduction device in use with the recording medium projects a laser beam converged by an objective lens to the information recording layer through the substrate and the layer, receives reflecting light from the recording layer, and generates a readout signal based on the reflecting light, the signal representing information readout of the recording layer, the laser beam passes through the layer having a uniform refractive index. Accordingly, the reflecting light received does not contain noise caused by different refractive indices, and hence it stably and exactly generates the readout signal representative of the information read out of the recording layer.
In a seventh information recording medium, which depends from any of the first to fifth recording media, the hard material consists of a medium whose refractive index is different from that of the base material, and is uniformly distributed in the base material, to thereby form the layer.
In the seventh recording medium, when the pickup of the reproduction device in use with the recording medium projects a laser beam converged by an objective lens, for example, to the information recording layer through the substrate of the recording medium rotating at a predetermined velocity and the layer, receives reflecting light from the recording layer, and generates a readout signal based on the reflecting light, the signal representing information readout of the recording layer, the laser beam of the pickup successively irradiates the hard material whose refractive index is different from that of the base material. Accordingly, noise is generated at the timings that the laser beam passes through the hard material. The noise generation timing is proportional to a radial position of the pickup engaging in reading information from the recording layer. Therefore, one can know the present radial position of the pickup in a manner that the reproduction device detects an average noise generation period in the readout signal.
In an eighth information recording medium, which depends from any of the first to fifth recording media, the hard material consists of a medium whose refractive index is different from that of the base material, and a mixing ratio at which the hard material is mixed into the base material is selected to be within an error correction ability of an error correction portion contained in a reproduction device which optically reads out information from the information recording medium and reproduces the same.
In the eighth recording medium, when the reproduction device optically reads out information from the information recording layer through light passing through the layer containing the hard material whose refractive index is different from that of the base material, even when the operation of reading out the recording information by the reading means is interrupted by the hard material, and the readout signal is consequently missed thereat, the error correction portion satisfactorily carries out an error correction process on the readout signal suffering from the missing, whereby the recorded information is reproduced without any missing of the signal.
A ninth reproduction device reproduces information from the eighth information recording medium, and comprises:
reading means for reading information from the information recording medium and generating a readout signal; and
a digital demodulator for digitally demodulating the readout signal by a predetermined method and correcting an error contained in the readout signal by an error correction portion contained therein, to thereby generate coded data based on the readout signal;
wherein even when the operation of reading out the recording information by the reading means is interrupted by the hard material, and the readout signal is consequently missed thereat, the error correction portion carries out an error correction process on the readout signal suffering from the missing, whereby the coded data corresponding to the readout signal is reproduced without any missing of the signal.
In the ninth recording medium, even when the operation of reading out the recording information by the reading means is interrupted by the hard material during the operation of reading out information from the information recording layer by the reading means, and the readout signal is consequently missed thereat, the error correction portion of the digital demodulator satisfactorily carries out an error correction process on the readout signal suffering from the missing, whereby the coded data corresponding to the readout signal is reproduced without any missing of the signal. Therefore, a reliable reproduction of the recording signal is ensured.